Adapter Pattern is used when we have to make a relationship between two incompatible interfaces so that they can work together.

Assume that you are creating an application for weather forecasting. Your application collects weather information from several sensors in a proprietary binary format, which you display on a user-friendly interface. The following figure shows different components of this application.

You choose to incorporate an advanced, third-party weather forecast system into your app to increase its accuracy. However, there is a twist: this method only accepts data in ordinary XML format, not the binary format provided by your sensors. The discrepancy in the data formats presents a problem.

Can you think of some solution to this problem? The solution is to create an Adapter. Here we go!

Here is a simple pictorial illustration showing a real-world example of the Adapter Pattern. The picture shows a view of a diplomatic meeting in which two diplomats, from different countries, speaking different languages, are communicating with each other with the help of a translator. Here, the translator translates the speech from both diplomats and acts as an adapter between two incompatible persons. This enables effective communication between the two parties despite their language barrier, just like the Adapter Pattern allows software components with incompatible interfaces to work together.

Structure of Adapter Pattern

The structure of the Adapter pattern can be shown using a class diagram. The components of class diagram are:

• Client: The class that contains the actual business logic. It interacts with the Adapter to use the service.
• Target Interface: This interface defines how the client wants to use the service. The Adapter class implements this interface.
• Adapter: This class implements the Target interface. It also contains a reference to the Adaptee class. It translates the interface from the Target to something the Adaptee understands.
• Adaptee: This is the class that implements the actual service that the Client wants to use but can't use directly because it has an incompatible interface with the Client.

Implementation of Adapter Pattern

Pseudocode

Let's implement the previously mentioned translator example and have a look at its pseudocode.

// Adaptee
CLASS FrenchSpeaker
    METHOD speakFrench(message)
        PRINT "Speaking in French: " + message
    END METHOD
END CLASS

// Target Interface
INTERFACE EnglishSpeaker
    METHOD speakEnglish(message)
END INTERFACE

// Adapter
CLASS Translator IMPLEMENTS EnglishSpeaker
    PRIVATE frenchSpeaker: FrenchSpeaker

    CONSTRUCTOR Translator(frenchSpeaker: FrenchSpeaker)
        this.frenchSpeaker = frenchSpeaker
    END CONSTRUCTOR

    METHOD speakEnglish(message)
        frenchMessage = translateToFrench(message)
        frenchSpeaker.speakFrench(frenchMessage)
    END METHOD

    PRIVATE METHOD translateToFrench(message)
        // Simplified translation logic
        RETURN message with "Hello" replaced by "Bonjour" and "Thank you" replaced by "Merci"
    END METHOD
END CLASS

// Client
CLASS EnglishClient
    PRIVATE speaker: EnglishSpeaker

    CONSTRUCTOR EnglishClient(speaker: EnglishSpeaker)
        this.speaker = speaker
    END CONSTRUCTOR

    METHOD express(message)
        speaker.speakEnglish(message)
    END METHOD
END CLASS

// Main
MAIN
    frenchSpeaker = NEW FrenchSpeaker()
    translator = NEW Translator(frenchSpeaker)
    client = NEW EnglishClient(translator)

    client.express("Hello! Thank you for the meeting.")
END MAIN

In this example, the Adapter Pattern is used to bridge the gap between the EnglishClient (which expects to communicate in English) and the FrenchSpeaker (which only understands French). The Translator acts as the adapter, translating English to French, allowing these two components to work together seamlessly. This demonstrates the power of the Adapter Pattern in integrating systems with incompatible interfaces.

• FrenchSpeaker is the Adaptee, providing functionality in French.
• EnglishSpeaker is the Target Interface.
• Translator is the Adapter, converting English messages to French.
• EnglishClient is the Client, using the services of EnglishSpeaker.

The Translator adapts the English messages to French, allowing the English-speaking client to effectively communicate with the French speaker.

Now, we will look at the implementation of this example in different programming languages.

Implementation